The invention herein described pertains generally to strain gauge apparatus, and more specifically to signal conditioning circuitry for semiconductor strain gauges.
It is well known to use strain sensitive resistors as transducers for sensing a variety of parameters. Typically one or more variable resistance elements are applied to a web or diaphragm which is subject to stress and/or strain produced by external conditions. Transfer of the strain to the resistance elements causes their resistance to change. These resistance changes can be detected by a variety of means, typically including bridge circuitry.
A relatively recent improvement in strain gauge transducers is the semiconductor strain gauge wherein the web or diaphragm comprises crystalline semiconductor material of one type (e.g., silicon doped with boron). This material forms a substrate into which p-type piezo-resistive elements can be diffused.
Silicon resistor strain gauges as described offer several important advantages over the more traditional wire strain gauges. One such advantage is the high sensitivity of a silicon strain gauge, typically in the order of 100 times that of a wire strain gauge. This large gauge factor does not depend on a change in geometry, but depends on a resistivity change caused by the strained crystal lattice. Conversely, other types of strain gauges, such as wire strain gauges, have gauge factors which depend on a change in geometry.
A second advantage is that silicon strain gauges can be diffused in a homogeneous crystalline silicon medium, thus providing sensing elements integrated into a silicon load bearing member. This avoids problems in bonded strain gauge apparatus resulting from bond degredation, as well as temperature sensitivity and hysteresis caused by the thermo-elastic strain.
A third advantage is that a silicon crystal is a superior force sensing material in that it does not permanently deform or yield.
A fourth advantage is that silicon strain gauges can be fabricated using common integrated circuit process technology.
A fifth advantage is that silicon diffused piezo-resistors can be manufactured in conventional integrated circuits to form true integrated circuit transducers, thus permitting a single chip of silicon to support both sensing and signal conditioning functions.
A strain sensor of the previously described type is disclosed in U.S. Pat. No. 3,853,650 issued Dec. 10, 1974 to Jerome T. Hartlaub, and assigned to the assignee of the present application. The patent further discloses an improved method for producing such silicon semiconductor strain sensors.
It has been common to determine stress and/or strain induced changes in resistivity of variable resistances transducer elements by means of bridge circuits. Representative of such arrangements are the systems shown in U.S. Pat. Nos. 3,457,493 and 3,836,796 issued to W. E. Shoemaker et al. on July 22, 1969 and J. E. Solomon et al. on Sept. 17, 1974 respectively. Both patents disclose systems in which strain sensitive resistors are connected to form Wheatstone bridge arrangements.
In order to achieve initial balance of a strain gauge bridge circuit it is normally necessary to place trimming resistors in series with or across the strain sensitive resistors. It is also highly desirable that the arms of the bridge exhibit effectively identical temperature response characteristics. This is particularly true in connection with piezo-resistive bridges because of the very large temperature dependence of piezo-resistive elements. However, it has been found difficult to simultaneously match the temperature coefficients and resistance values of the bridge arms.
The system of previously noted U.S. Pat. No. 3,457,493 discloses an arrangement for overcoming this problem by employing separate current sources in different arms of the bridge arrangement. A disadvantage of such a system is the requirement for more than one current source.
The advantages of maintaining a constant current through the strain sensitive resistors, while avoiding the complications of the system of U.S. Pat. No. 3,457,493, is disclosed in U.S. Pat. No. 3,841,150 issued Oct. 15, 1974 to the same inventor and assigned to the same assignee as the present application. A principal feature of the system of U.S Pat. No. 3,841,150 is that two strain gauge resistors are biased by operational amplifiers in a manner which maintains a constant current flow through the resistors. A portion of the measurement signal is fed back to the operational amplifiers in order to increase response linearity. A temperature correction circuit maintains the ratio of the change in output signal relative to the change in input signal at a substantially constant value over a wide range of temperatures.
The circuit of U.S. Pat. No. 3,841,150 has been found to provide excellent performance in demanding and critical applications, such as in military aircraft systems. However, there are a number of less critical applications wherein performance requirements are not as demanding and wherein simplicity and low cost are exceptionally important. The circuit of U.S. Pat. No. 3,841,150 tends to be overly complex and expensive for such applications.
In line with the requirement simplicity and low cost, the applicant has discovered a unique simplified strain gauge system in which satisfactory performance is achieved with extremely simple signal conditioning circuitry. The advantages of constant current exitation of the strain sensitive resistors are maintained. Substantially constant current flow through the strain sensitive resistors is achieved with basically only a single operational amplifier and a minimum number of other noncritical electronic components.